The invention relates to high definition television (HDTV) and to spread spectrum communication systems.
The United States is in the process of setting a standard for high-definition television transmission in the same terrestrial broadcasting (over-the-air) channels now used for transmitting standard (known in the US and some other countries as NTSC) television. This plan is quite different from the intentions in Europe and Japan, in which HDTV will be delivered only by satellite transmission (DBS).
There are two important differences that affect the system design. DBS provides a "clean" channel, essentially free of ghosts and interference, and with a guaranteed minimum signal level. Terrestrial broadcasting is typically impaired by ghosts, noise, interference, and frequency distortion. These characteristics and impairments set the practical limit to picture quality in typical homes, and they also affect the number of channels that can be used in any one area (10 to 20) as compared with the total number of channels allocated for television (about 68). In addition, terrestrial spectrum is in very short supply. There are many more claimants for spectrum assignments than can be accommodated by the regulatory authorities. Consequently, these authorities are very much concerned with the spectrum efficiency of proposed HDTV systems. For this application, spectrum efficiency refers to the number cf different TV programs of a given technical quality that can be provided to each viewer within a given overall allotment of spectrum.
It is inherent in terrestrial broadcasting, due to the operation of the inverse-square law, that the signal power level, and therefore the CNR (carrier/noise ratio), drops rapidly as one goes further from the transmitter. Since the theoretical capacity of the channel to deliver information is proportional to the bandwidth times the CNR (in dB), it is necessary to deliver more information, and hence a better picture, to close-in receivers in order to achieve efficient utilization of channel capacity. All current analog systems do this as a matter of course. Systems that do not have a graceful degradation of image quality with worsening CNR inherently waste bandwidth in the central cities, precisely where a spectrum shortage is developing due to the rapid growth of mobile services.
Recently, several all-digital HDTV systems have been proposed. All of them deliver about 20 Mb/s to all viewers, and all have a very sharp threshold, below which no reception at all is obtained. This sharp threshold is due to the very rapid increase in bit error rate (BER) as a function of CNR. Typically, the BER increases by a factor of 10 with a 1-dB drop in CNR. No known scheme of error protection can overcome this effect and still maintain good transmission efficiency.
An alternative is to use some form of progressive transmission in which the transmitted signal is divided into a number of data streams that are transmitted at different effective power levels, using frequency division (FD) or time division (TD). The higher the CNR, the more data streams are recovered, and the better the picture. While these methods are in the right direction, they are not very efficient, since, at most levels of CNR, one or more of the data streams is being delivered at excess CNR, and therefore wastefully. In addition, since this class of schemes uses FD or TD, there is a fixed upper limit of image quality, and there is no convenient way to upgrade the system over time.